Heat sealing pressure monitoring system

ABSTRACT

The method provides for monitoring pressure applied by a heat sealing tool. The heat sealing tool seals a lidstock to the flange of a rigid polymeric container with a well containing a contact lens and an aqueous storage solution. The flange includes an upper sealing surface and a lower surface. The tool includes a first plate adapted to contact the lower surface of the flange and a second plate adapted to press the lidstock against the sealing surface of the flange.

FIELD OF THE INVENTION

The invention relates to heat sealing tools for sealing lidstock to plastic containers to form sealed packages, particularly disposable packages for contact lenses known as blister packages. Especially, this invention relates to monitoring the pressure applied in sealing lidstock to such plastic containers.

BACKGROUND OF THE INVENTION

Soft hydrogel contact lenses have been increasing in popularity ever since they were first introduced in the 1970's. Such contact lenses are conventionally packaged in the hydrated state and in a storage solution. Many contact lenses are packaged, along with the aqueous storage solution, in a disposable blister package typically formed from a rigid polymer. Examples of blister packages can be seen in U.S. Pat. Nos. 4,691,820; 5,524,419; 5,578,331, 5,649,410, 5,722,536 and 6,082,533. The base of a blister package can either be part of the original contact lens mould or a separately moulded base, typically formed from a rigid polymer. In the above mentioned package types, the base that contains the contact lens and any aqueous storage solution, is sealed by a flexible lidstock. This lidstock is typically a laminated foil which can be pulled back by a user to access the lens contained in the base. Typically, this lidstock is sealed to the area surrounding the portion of the base containing the contact lens. This sealing area of the base has typically been a substantially flat surface area, or a raised flattened annular surface as seen, for example, in U.S. Pat. No. 5,722,536.

SUMMARY OF THE INVENTION

The sealing tool for sealing the lidstock to the flange of a plastic container typically has a lower face which supports the container directly below the flange, and an upper face which presses the lidstock against the surface sealing surface of the flange of the package, supported by the lower face which is in contact with the lower surface of the flange of the package. Typically, the upper face is heated in order to provide a seal by melting a thermoplastic basal layer of the lidstock to the sealing surface of the flange.

This invention provides a method of monitoring the pressure applied to an individual blister package during heat sealing of lidstock to the flange of the package.

The sealing tools may have flat faces where they contact the lidstock and lower surfaces of the flange of the package for sealing. Alternately, and according to preferred embodiments, the sealing tool may have curved faces, for example, with substantially the same curvature as the sealing surface of the flange of the package.

The invention will now be further described, by way of example only, with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded view of a tool according to the invention with a sealed blister pack between the plates of the tool. The curvature of the surfaces has been exaggerated for purposes of illustration.

The lower first plate 1 of the tool has a recess 2 in the first face 8 to accommodate well 7 of the blister pack as sealing takes place. The lidstock 5 is in contact with the upper sealing surface of the flange 4 of blister pack 6. The first face 8 is curved with the same curvature as the lower surface of the flange 4. The second face 9 of the upper second plate 3 of the tool is curved with the same curvature as the upper sealing surface of the flange 4.

Accordingly, lower first part I of the tool serves to elevate the blister pack so that the lidstock 5 contacts the second face 9 of upper second part 3. Lower first part 1 includes a servo driven block 10 to elevate the blister pack. This lower first part 1 further includes spring 11 and pressure transducer 12. Pressure transducer 12 is connected to a controller, shown schematically as 13, so that pressure the applied pressure, detected by transducer, can be monitored. If desired, the controller 13 may be further connected to a display so that an operator can observe real-time operation of the heat sealing unit.

It may be desired to heat seal several blister packs concurrently, with the blister packs arranged in an array. However, when this is done, it is preferred that the heat sealer for each blister pack is monitored individually.

Detailed Description of Preferred Embodiments

According to preferred embodiments of this invention, the blister package has the form of a rigid polymeric container comprising a well and a flange. The flange has a curved upper sealing surface and a lower surface. The heat sealing tool comprises a first plate adapted to contact the lower surface of the flange and a second plate adapted to press the lidstock against the sealing surface of the flange, wherein the second plate is curved with substantially the same curvature as the sealing surface of the flange. Preferably, the first plate is also curved with substantially the same curvature as the lower surface of the flange, as illustrated schematically in FIG. 1. Ideally the second plate is curved with an identical curvature as the sealing surface of the flange and the first plate is curved with an identical curvature as the lower surface of the flange.

By providing the second plate with substantially the same curvature as the curved sealing surface of the flange, when the second plate comes into contact with the lidstock to press the lidstock against the sealing surface of the flange, substantially uniform contact time and contact pressure will be delivered over the sealing surface of the flange, leading to greater uniformity in the strength of the seal over the entire flange. This uniformity may be further improved by providing the first plate with substantially the same curvature as the lower surface of the flange, such that as the plates are squeezed together sandwiching the lidstock and the flange, substantially uniform pressure is applied over the entire sealing surface of the flange over the entire time span of the sealing process.

Suitably, in use, the first plate is not heated and is at ambient temperature, while the second plate is heated to a suitable temperature such that a hermetic seal is formed by heat transmitted through the lidstock when the plates are brought together to sandwich the lidstock and flange.

The lidstock is suitably a laminated foil with a basal layer which melts to form a seal when heated in contact with the sealing surface of the flange. Preferably, the upper layer of the laminated foil is a surface upon which printing may be carried out. A suitable upper layer is a metal foil such as aluminium foil.

The container is suitably moulded from a rigid polymer, for example a thermoplastic polymer such as polypropylene or polystyrene. The container comprises a well for holding a soft contact lens immersed in an aqueous, storage solution, and has a flange around the well to which the lidstock is sealed to form the sealed package. The flange may simply be the top surface of the package itself, or may be in the form of a raised area surrounding the well and provided on the top surface of the package. The sealing surface of the flange may be all or part of the upper surface of the flange, provided the sealing surface surrounds the well such that when a seal is made, the contents of the well are hermetically sealed. The nature of the moulding process can lead to the top surface of the package and hence the flange, and the sealing surface of the flange, having a concave or a convex upper surface. This may arise from uneven shrinkage of the polymer forming the container during the moulding process to form the well.

Where the container starts as a flat sheet of polymer and then is moulded simply to form a well, leaving the remaining surface as the flange of the well, the upper sealing surface of the flange and the lower surface of the flange will be endowed with the same curvature as a result of the uneven tensions set up in the moulding process, but with the upper surface being concave and the lower surface convex or vice versa.

In order to better control the strength of the seal made by heat sealing tools according the invention, it is preferred the tools are calibrated such that when the first and second plates are brought together to sandwich the lidstock and the flange of the container, the force applied is controlled within a preferred range. Such a calibration may be achieved by means of pressure or displacement readings on the plates, and associated calculation formulae. However, a preferred method is to use a strain sensing load cell (such as a semiconductor load cell) mounted into a replica blister package with the surface of the load cell at the same position as the sealing surface of the blister package to be used with the tool of the invention. The replica blister package is mounted onto the first plate and then the first and second plates are brought together to their normal sealing position and the applied force shown by a display monitor connected to the load cell. The normal sealing position may then be adjusted and set to give the required sealing force as shown by the display.

The method of the present invention is directed not to such calibration operations, but to monitoring the pressure applied to individual blister packs and their lidstock during the heat sealing process. In other words, the method of this invention allows for monitoring the heat sealing process to ensure that the desired, proper pressure, determined initially by calibration, was applied in sealing a blister package. It will be appreciated that the heat sealing unit is used to sequentially seal multiple blister packs. Over time, a heat seal unit may fail to apply this desired, proper pressure, whereas this invention allows for ensuring that the desired pressure was applied. And because the container includes a sterile storage solution, it is important that a hermetic seal is obtained for each package.

This invention provides several practical applications. As an example, the controller may be programmed to set off an alarm if a pressure within a predetermined range was not applied during the heat sealing operation. Accordingly, such an alarm serves to alert an operator to check integrity of the heat seal applied to that package and/or to confirm whether maintenance of the heat sealing unit is required. 

1. A method comprising monitoring pressure applied by a heat sealing tool, wherein said heat sealing tool seals a lidstock to a rigid polymeric container comprising a well containing a contact lens and a storage solution, and a flange with an upper sealing surface and a lower surface, wherein the tool comprises a first plate adapted to contact the lower surface of the flange and a second plate adapted to press the lidstock against the sealing surface of the flange.
 2. The method according to claim 1, wherein the first plate elevates the container and the second plate is heated.
 3. The method according to claim 2, wherein the first plate is in connection with a pressure transducer that measures pressure applied by the first and second plates to the container flange and lidstock.
 4. The method according to claim 3, wherein the pressure transducer is connected to a controller.
 5. The method according to claim 1, wherein the second plate is curved with an identical curvature as the sealing surface of the flange.
 6. The method according to claim 1, wherein the first plate is curved with substantially the same curvature as the lower surface of the flange.
 7. The method according to claim 1, further comprising setting off an alarm if the monitored pressure is not within a predetermined range during the heat sealing operation. 